Inkjet printing mechanisms may be used in a variety of different products, such as plotters, facsimile machines and inkjet printers, to print images using a colorant, referred to generally herein as "ink." Inkjet printing mechanisms typically have a printhead which is propelled from side to side across a print media, such as paper, with the printhead being controlled to selectively deposit ink in a desired pattern on the page. Some inkjet print mechanisms carry an ink cartridge with a full supply of ink back and forth across the sheet. Other inkjet print mechanisms, known as "off-axis" systems, propel only a small ink supply with the printhead cartridge across the print zone, and store the main ink supply in a stationary reservoir, which is located "off-axis" from the path of printhead travel. Typically, a flexible conduit is used to convey the ink from the off axis main reservoir to the printhead cartridge. In multi-color cartridges, several printheads and reservoirs are combined into a single unit, with each reservoir/printhead combination for a given color being referred to as a "pen."
In the past, inkjet pens have been arranged in a side-by-side fashion, for example, as shown schematically in FIG. 5 for a multi-color cartridge 200. The cartridge 200 has pens are arranged in a one-by-four matrix, side-by-side and parallel to a scanning axis, as indicated by arrow 201. The scanning axis 201 defines the path of travel of the printhead carriage over the print zone. The cartridge 200 has four pens, specifically black ("K"), magenta ("M"), yellow ("Y") and cyan ("C") pens 202, 204, 206 and 208, with a casing 210 defining pen reservoirs 212, 214, 216, 218, respectively. An orifice plate 220 may be used to define black, magenta, yellow and cyan ink-ejecting nozzle sets 222, 224, 226, 228 for the respective pens 202, 204, 206 and 208. Ink feed or inlet orifices, 232, 234, 236, 238 supply ink from reservoirs 212, 214, 216, 218 to the ink ejection mechanism (not shown) of the respective nozzle sets 222, 224, 226, 228.
Between each ink feed orifice 232, 234, 236, 238 and its associated nozzle set 222, 224, 226, 228 lies an ink ejection mechanism that may take on a variety of different forms known to those skilled in the art, for instance, using piezo-electric or thermal printhead technology. For purposes of illustration, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer (not shown) containing ink channels and vaporization chambers is located between the orifice plate 220 and a substrate layer (not shown). This substrate layer typically contains linear arrays of heater elements, such as resistors, which when energized, heat the ink within the vaporization chambers to eject an ink droplet from a discrete nozzle associated with the energized resistor. By selectively energizing the resistors, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
The minimum width of these earlier multi-pen assemblies is limited by the ink pressure regulation system feeding each group of nozzles. Typical ink pressure regulation systems are often constructed using foam, for instance, or using a resilient bladder system. In one typical earlier system, such as cartridge 200 of FIG. 5, the depth D.sub.1 of the casing 210 is about 45 mm, and each of the reservoirs 212-218 has a width of approximately 18.5 mm, with a spacing of 2.5 mm being required between adjacent reservoirs. Thus, the overall width W.sub.1 between the outer most edge of the black nozzle set 222 and the outer most edge of the cyan nozzle set 228 is about 66 mm. Using a typical spacing of 9.3 mm for distance between the two outermost reservoirs 222, 228 and the outboard edges of the casing 210, the overall width W.sub.2 of the pen casing 210 is about 100 mm. Even if the width of each pressure regulation system 222-228 is on the order of 15 mm, this arrangement makes it very difficult to feed ink toward the central line of the carriage, while providing a narrow column-to-column nozzle spacing. The wide column-to-column nozzle spacing of cartridge 200 decreases the throughput (e.g., pages per minute) of the printing mechanism because the printhead must traverse a longer path to scan each printhead over the entire print zone. Unfortunately, this longer scanning path also increases the product width.
Another system to minimize product width arranges the pens in a four-story stack, extending radially away from the axis, typically in a vertical direction. Such a vertical array suffers its own set of difficulties. For example, the printhead carriage must now be of a heavier construction to handle the moment of inertia created by such a top-heavy design. Also, the ink from the uppermost reservoirs if used infrequently, may be subject to drying and clogging within the feed passageways. Furthermore, the ink reservoirs of such a system are difficult to access for replenishing the ink supply. To accommodate a four-story pen stack, these products are usually taller than other products using pen 200, for instance, which detracts from the esthetic appeal of four-story pen units.
Thus, the earlier pen arrangement systems proposed have inadequately addressed the needs of increasing throughput and minimizing product width, as illustrated above with respect to an inkjet printer. Increased throughput, often measured in pages per minute, is preferred by consumers. Larger equipment is usually heavier and more costly to manufacture and ship, as well as being undesirable to some consumers who prefer more compact equipment with a smaller footprint, i.e. requiring a smaller area to rest upon a work surface or desk.